PROJECT SUMMARY ABSTRACT Substantial therapeutic advances have been made in NSCLC subsets harboring specific genomic alterations and targetable with small molecule kinase inhibitors. Unfortunately, a similar strategy has been unsuccessful for the ~30% of patients with mutated KRAS. Similarly, immune checkpoint inhibitors of the PD-1/PD-L1 axis provide durable response to ~20% of NSCLC patients, but the majority of patients do not benefit from this single-agent approach. There is a knowledge gap about the regulation of MAPK pathway signaling in mutant KRAS tumors, and the interplay between oncogenic signaling and the immunosuppressive microenvironment, which translates into a major unmet therapeutic need. The members of our multidisciplinary team (Gibbons, Heymach, Wistuba, Draetta and Wang) have a track record of productivity in studying KRAS mutant lung cancer and represent expertise in mouse modeling of human lung cancer, clinical oncology, immunotherapy, molecular pathology of lung cancer, drug development and bioinformatics. The Investigators have developed preliminary data from analysis of human lung cancer specimens and preclinical Genetically Engineered Mouse Models (GEMMs) of KRAS mutant NSCLC that the epithelial-mesenchymal transition (EMT) status of tumor cells is critical to their therapeutic response to MEK inhibitors, with the epithelial state producing profound sensitivity to MEK inhibitors and the mesenchymal state producing resistance, even in mutant KRAS tumors. Further, we have published that the microRNA-200-ZEB1 axis regulates EMT, the immune microenvironment of tumors and subsequent response to immune checkpoint inhibitors. Based upon preliminary data, we hypothesize that: 1. Tumor cell EMT produces heterogeneity in KRAS mutant tumors by suppressing MAPK pathway signaling, 2. The altered tumor immune microenvironment resulting from tumor cell EMT confers targetable vulnerabilities to new immune therapies, 3. Combination immune checkpoint inhibitors and signaling pathway inhibitors will provide an effective complementary targeting strategy for mutant KRAS NSCLC. We will address these hypotheses by: i) evaluating the role of EMT in de novo and acquired resistance to MEK inhibitors in preclinical models of lung adenocarcinoma, ii) determining the effects of MEK inhibitors on the tumor immune microenvironment and testing the efficacy of their combination with immune checkpoint inhibitors to enhance response in preclinical models of KRAS mutant lung adenocarcinoma, and iii) characterizing the relationship of EMT to MAPK pathway activation in human lung cancer samples, and the markers of sensitivity/resistance to combination anti-PD-L1/MEK inhibitor treatment in clinical trial specimens.